White matter (WM) injury, which is characterized by axonal degeneration and loss of the myelin sheath (demyelination), is important for the long-term functional deficits after traumatic brain injury (TBI). The central nervous system exhibits limited capacity for WM repair, such as axonal regeneration and remyelination. During post-TBI WM repair, oligodendrocyte precursor cells (OPCs) are known to actively proliferate. However, many newly generated OPCs fail to differentiate into mature, myelin- This application will examine the effect of intranasal delivery of protease inactive plasminogen activator mutant (tPAm) to enhance differentiation and maturation of oligodendyocytes to promote white matter integrity following TBI. producing oligodendrocytes (OLs), resulting in inadequate remyelination. Failed remyelination not only diminishes signal transduction, but also leads to axon degeneration and worsens clinical outcome. Thus, interventions that promote OPC differentiation and maturation are promising strategies to enhance WM repair and improve functional recovery. Recombinant tissue plasminogen activator (tPA) is an FDA-approved treatment for ischemic stroke and catalyzes thrombolysis through serine protease action. Recent studies have discovered direct neurorestorative effects of tPA independent of protease activities. However, the clinical use of tPA as a therapeutic agent in TBI raises several concerns, as it can also cause blood-brain barrier damage and brain edema. In this proposal, we will explore protease-inactive mutant tPA (tPAm), with substitution of a single amino acid (S478A) to eliminate protease action, as a novel restorative therapy to promote remyelination and WM repair after TBI. We discovered that nanomolar concentrations of tPAm promote the differentiation of cultured primary OPCs into mature OLs. In addition, tPA knockout exacerbates behavioral deficits and WM injury lasting at least 35 days after TBI whereas post-injury intranasal administration of tPAm improves long-term neurological function and WM integrity after TBI in mice. Pilot data further suggest that tPAm enhances WM repair after TBI by promoting OPC differentiation and axon remyelination and that the effect may be mediated by the peroxisome proliferator-activated receptor ? (PPAR?) nuclear receptor. Here we will focus on the novel remyelinating actions of tPAm and test the following hypothesis: Treatment with protease inactive tPAm facilitates WM repair and long-term neurological recovery after TBI, at least in part, by inducing OPC differentiation and axonal myelination through PPAR? activation. Three Specific Aims will be tested. Aim 1: Determine whether post-TBI treatment with tPAm enhances WM integrity and promotes long-term recovery. Aim 2: Test the hypothesis that tPAm induces OPC differentiation/maturation and promotes axonal myelination through PPAR? activation. Aim 3: Test the hypothesis that tPAm-induced OPC differentiation and axonal myelination are essential for the protection of WM integrity and long-term recovery after TBI. These studies are the first to investigate the potential for tPAm to foster remyelination in TBI an will identify the underlying mechanism of action, thereby setting the stage for the potential use o tPAm in the clinic.